Spinal or vertebral rods are often used in the surgical treatment of spinal disorders such as degenerative disc disease, disc herniation, scoliosis or other curvature abnormalities or fractures. Spinal rods, which may be mechanically anchored to sequentially aligned pedicle screw assemblies connected to vertebral bodies, serve to provide rigidity to portions of the spinal column to encourage the vertebral bodies to fuse after spinal-fusion surgery. Fusion results in the permanent immobilization of one or more of the intervertebral joints between vertebral bodies. To achieve spinal fusion the spinal rods selected are typically uniform along the entire length of the rod and manufactured from a single or integral piece of relatively inflexible material having a uniform diameter and sized to provide substantially rigid support to the spinal construct.
Fusion, however, can have some very undesirable side effects. Spinal fusion by design results in immobilization of a portion of the spine and thus can severely limit the natural motion of the subject. Further, although fusion can result in a strengthened portion of the spine, it may also result in more rapid degeneration and even hyper-mobility and collapse of other portions of the spine that are adjacent to the portion of the spine being fused.
An alternative to the use of rigid spinal rods is the use of flexible or dynamic spinal rods to create a more normal loading pattern in flexion, extension, distraction, compression, side bending and torsion. The efforts to provide a dynamic spinal rod conventionally involve the use of flexible materials that are capable of providing the needed bending and twisting dynamics, but these materials can lack the necessary strength to avoid the damage that can result from the compressive forces of bone screw attachments to the spinal rods.
Recent attempts to provide dynamic spinal rods typically include the use of a rod formed of flexible plastic material, such as polyurethane, UHMW polyethylene, PEEK or Teflon. Efforts to include a measured degree of reinforcement to such flexible rods have employed longitudinally aligned reinforcing components that extend internally through the length of the flexible rods, the reinforcing components being formed of materials such as Kevlar, polyethylene, polyurethane, Teflon fiber, carbon fiber, or stainless steel. Common to all current attempts to provide a flexible spinal rod is the potential failure of such rods to provide flexibility while being sufficiently strong and resistant to the damaging compressive forces of attached bone screws.
There exists therefore a need to provide a flexible spinal rod that is capable of being secured with conventional bone screws without being susceptible to damage from the compressive forces at the attachment point of the bone screws.